Fuel cells using solid polymer electrolyte (SPE) membranes are expected to find widespread use as power supplies for electric cars and small-size auxiliary power supplies due to a low operating temperature below 100° C. and a high energy density. For such SPE fuel cells, constituent technologies relating to electrolyte membranes, platinum base catalysts, gas diffusion electrodes, and electrolyte membrane/electrode assemblies are important. Among others, the technology relating to electrolyte membranes is most important because they largely govern the performance of fuel cells.
In SPE fuel cells, an electrolyte membrane on its opposite sides is combined with a fuel diffusion electrode and an air diffusion electrode so that the electrolyte membrane and the electrodes form a substantially integral structure. Then the electrolyte membrane not only acts as an electrolyte for conducting protons, but also plays the role of a diaphragm for preventing a fuel (such as hydrogen or methanol) from directly mixing with an oxidant (such as air or oxygen) even under applied pressure.
From the electrolyte aspect, the electrolyte membrane is required to have a high ion (proton) transfer velocity, a high ion exchange capacity, and a high and constant water-retaining ability enough to maintain a low electric resistance. The role of a diaphragm requires the electrolyte membrane to have a high dynamic strength, dimensional stability, chemical stability during long-term service, and no extra permeation of hydrogen gas or methanol as the fuel and oxygen gas as the oxidant.
Electrolyte membranes used in early SPE fuel cells were ion exchange membranes of hydrocarbon resins obtained through copolymerization of styrene with divinyl benzene. These electrolyte membranes, however, lacked practical usefulness due to very low durability. Thereafter, perfluorosulfonic acid/PTFE copolymer membranes developed by E.I. duPont and commercially available under the trade mark “Nafion” have been widely used instead.
One problem associated with conventional fluororesin base electrolyte membranes as typified by Nafion is an increased cost because their manufacture starts from the synthesis of monomers and requires a number of steps. This becomes a substantial bar against practical applications. With respect to the thickness of electrolyte membranes, as the membrane becomes thinner, proton conduction becomes easier and hence, fuel cells provide better power generation characteristics. Thin electrolyte membranes, however, can be ruptured when an electrolyte membrane and electrodes are pressed together at elevated temperature to enhance the bond therebetween.
Efforts have been made to develop inexpensive electrolyte membranes that can replace the Nafion and similar membranes. There are known, for example, electrolyte membranes made of polyether ether ketone or similar hydrocarbon polymers having sulfonic acid groups introduced therein, and electrolyte membranes made of fluororesins to which styrene or similar aromatics are radiation grafted, with sulfonic acid groups being introduced into the aromatic rings. However, these electrolyte membranes after their film formation are joined to electrodes by pressing at elevated temperatures, which leaves problems of possible rupture of membranes and complex steps. The joining under heat and pressure does not always achieve a sufficient adhesion.
To improve the level of productivity and adhesion, JP-A 2003-203646 proposes to apply a solution of an electrolyte membrane in a solvent onto an electrode, and press bond the assembly with the solvent partially left therein. Since the electrolyte membrane has not been cured, only low adhesion is achieved.
JP-A 2003-217342 and JP-A 2003-217343 disclose crosslinking of electrolyte membranes for improved durability. Since solid electrolyte membranes are crosslinked, subsequent press bonding at elevated temperatures is necessary to fabricate an electrolyte membrane/electrode assembly.
Also, WO 03/033576 discloses to control the fuel permeability of an electrolyte membrane by impregnating the electrolyte membrane with a non-electrolyte monomer, followed by polymerization. The non-electrolyte monomer is cured. However, since the membrane subject to impregnation is in solid form, subsequent press bonding at elevated temperatures is necessary.